Binding energies for alkane molecules on a carbon surface from gas–solid chromatography and molecular mechanics

Abstract

Gas–solid chromatography was used to determine B 2 s (gas–solid virial coefficient) values for 12 alkanes (10 branched and 2 cyclic) interacting with a carbon powder (Carbopack B, Supelco). B 2 s values were determined by multiple size variant injections within the temperature range of 393 to 623 K with each alkane measured at 5 or 6 different temperatures. The temperature variations of the gas–solid virial coefficients were used to find the experimental adsorption energy or binding energy values ( E ∗ ) for each alkane. A molecular mechanics based, rough-surface model was used to calculate the molecule–surface binding energy ( E cal ∗ ) using augmented MM2 parameters. The surface model consisted of three parallel graphene layers with each layer containing 127 interconnected benzene rings and two separated nanostructures each containing 17 benzene rings arranged in a linear strip. As the parallel nanostructures are moved closer together, the surface roughness increases and molecule–surface interactions are enhanced. A comparison of the experimental and calculated binding energies showed excellent agreement with an average difference of 3.8%. Linear regressions of E ∗ versus E cal ∗ for the current data set and a combined current and prior alkane data set both gave excellent correlations. For the combined data set with 18 linear, branched and cyclic alkanes; a linear regression of E ∗ = 0.9848 E cal ∗ and r 2 = 0.976 was obtained. The results indicate that alkane-surface binding energies may be calculated from MM2 parameters for some gas–solid systems.